Agar Plate Methods for Assessing the Antibacterial Activity of Thyme and Oregano Essential Oils against S. epidermidis and E. coli

The resistance to antimicrobials (AMR), especially antibiotics, represents a serious problem and, at the same time, a challenge. In the last decade, a growing interest in the use of essential oils (EOs) as antimicrobial substances was observed. Commercial thyme and oregano EOs are reported to be the main responsible of the oil antimicrobial efficacy against both Gram-positive and Gram-negative pathogenic bacteria. The aim of the present work was to study the efficacy of EOs against Staphylococcus epidermidis and Escherichia coli in long-time treatments. In a preliminary microdilution test, a MIC value was obtained for thyme EO against S. epidermidis and E. coli. After that, disk diffusion and disk volatilization tests were set up to study the influence of main cultural parameters on EO activity in liquid or vapor phase. Both bacteria were inhibited by thyme and oregano EOs when applied pure (100% v/v) or diluted (75% and 50% v/v): a higher inhibition was observed in a disk diffusion test in which the antimicrobial effect was due to both liquid and vapor phase components. Finally, a comparison with literature data was carried out even if it was not so easy because standard methods are usually modified and adapted to specific case study. For this reason, the results have to be interpreted in relation to the analytical method applied.